1. Field of the Invention
The present invention relates to turbine engine arrangements and more particularly to a contra rotating turbine engine arrangement.
2. Description of Related Art
Operation of turbine engines is well known. Referring to FIG. 1, a gas turbine engine is generally indicated at 10 and comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high pressure compressor 14, a combustor 15, a turbine arrangement comprising a high pressure turbine 16, an intermediate pressure turbine 17 and a low pressure turbine 18, and an exhaust nozzle 19.
The gas turbine engine 10 operates in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 which produce two air flows: a first air flow into the intermediate pressure compressor 13 and a second air flow which provides propulsive thrust. The intermediate pressure compressor compresses the air flow directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high pressure compressor 14 is directed into the combustor 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive, the high, intermediate and low pressure turbines 16, 17 and 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low pressure turbines 16, 17 and 18 respectively drive the high and intermediate pressure compressors 14 and 13 and the fan 12 by suitable interconnecting shafts.
In order to improve engine operational efficiency it is known to provide in the low pressure turbine stages, that is to say equivalent to stages 17, 18 in FIG. 1, contra rotating turbines to avoid the necessity of providing stator elements for improved efficiency between those stages. Clearly, in order to achieve such contra rotation it is necessary to create contra rotation in the shafts upon which the turbine blades are secured or through securing those blades to casings such that there is contra rotation between those casings and/or shafts of adjacent turbine blades. In these circumstances it will be appreciated that relative compromise must be made between the benefits of eliminating the greater weight of a stator at an intermediate position between turbine rotor blade stages and problems associated with achieving such contra rotation.